Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. For blood flow measurements, returning ultrasonic waves are compared to a frequency reference to determine the frequency shifts imparted to the returning waves by moving objects including the vessel walls and the red blood cells inside the vessel. These frequency shifts translate into velocities of motion.
In state-of-the-art ultrasonic scanners, the pulsed or continuous wave Doppler waveform is com- puted and displayed in real-time as a gray-scale spectrogram of velocity versus time with the gray-scale intensity (or color) modulated by the spectral power. The data for each spectral line comprises a multiplicity of frequency data bins for different frequency intervals, the spectral power data in each bin for a respective spectral line being displayed in a respective pixel of a respective column of pixels on the display monitor. Each spectral line represents an instantaneous measurement of blood flow.
In the conventional spectral Doppler mode, an ultrasound transducer array is activated to transmit by a transmit ultrasound burst which is fired repeatedly at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. The return radiofrequency (RF) signals are detected by the transducer elements and then formed into a receive beam by a beamformer. For a digital system, the summed RF signal from each firing is demodulated by a demodulator into its in-phase and quadrature (I/Q) components. The I/Q components are integrated (summed) over a specific time interval and then sampled. The summing interval and transmit burst length together define the length of the sample volume as specified by the user. This so-called "sum and dump" operation effectively yields the Doppler signal backscattered from the sample volume. The Doppler signal is passed through a wall filter, which is a high pass filter that rejects any clutter in the signal corresponding to stationary or very slow-moving tissue, including a portion of the vessel wall(s) that might be lying within the sample volume. The filtered output is then fed into a spectrum analyzer, which typically takes the complex Fast Fourier Transform (FFT) over a moving time window of 64 to 256 samples. The data samples within an FFT analysis time window will be referred to hereinafter as an FFT packet. Each FFT power spectrum is compressed and then displayed via a gray map on the monitor as a single spectral line at a particular time point in the Doppler velocity (frequency) versus time spectrogram.
Typically the I and Q components of the Doppler signal are filtered separately by identical wall filters, which can be implemented as either an FIR or IIR filter. For s harp rejection of low-frequency clutter, a narrow transition band in the frequency response of the filter is required. Typically, the wall filter cutoff frequency is manually selected via a front-panel control key. Usually the wall filter cutoff frequency is increased when bright, low-frequency clutter is seen in the spectral image. Each time the wall filter cutoff setting is changed, a corresponding set of filter coefficient values are read out of a lookup table (LUT) and loaded into the wall filters.
The main limitation with the manually selected filter approach in the prior art is that once the cutoff frequency is set, the wall filter does not change even though the clutter frequency and bandwidth may vary with time, due to radial and/or lateral motion of the vessel walls over the cardiac cycle. As a result, the selected filter cutoff is often optimal only for a small portion of the cardiac cycle.